Most of the bicycles come with an engagement system, which is almost always is an integral part of the rear wheel, or to be precise – of the rear hub. There are some exceptions – like track, or fixed gear hubs and trial bikes. The latter often employ the engagement system not as part of the hub, but of the bicycle frame and is integrated with a bottom bracket unit.
The purpose of the engagement system is to temporarily disconnect the drive system from the wheel. That is when a rider stops pedalling, the wheel continues to roll. This mode is also known as “coasting”. Technically, it enables the one-way power transmission from the cranks to the wheel. One-way means that the rotation motions is always transferred from the cranks to the wheel, and not the other way around. All engagement systems automatically switch between engaged and disengaged mode. The system disengages as soon as the rotational speed of the wheel and of the drive unit do not match. The drive unit, or rotor, on the most road and MTB bikes is commonly referred as the freehub. It mounts the sprocket or cassette and receives the rotational motion from the cranks.
What are qualities of the engagement systems that we care about? For most riders that would be the efficiency, reliability and weight. All three are interdependent, and the gain in one aspect often comes with the trade-off in another. Let us briefly review them one by one.
How quickly the hub engages, that is what is the time, or distance that crank’s arm travels before the motion gets transferred to the wheel. It is about efficiency of the system. The longer it takes for the system to switch into the engaged mode, the more of the rider’s power is used ineffectively and is not contributed towards the primary purpose – rotating the wheel. The second attribute of how efficiently the engagement system operates is about its internal loses, such as losses to the friction.
For road bikes the internal losses are the significant factor, whilst the speed of the engagement is relatively less important, compared with MTB. It is because more challenging off-road terrain demands often changes between gears and between pedaling and coasting.
One way to measure how quickly system engages is to equate it with the angle of the circle that cranks travels, before their rotation is transferred to the wheel. It is not fixed and precise measure, as the what is gear is used will affect it in a broad range. At the same time, hub manufacturers state that parameter, often named as points of engagements or POE. The number of POE gives the measure in degrees of an angle with the simple formula: degrees of the engagement is equal to 360 divided by the number of POE. For example, Chris King’s R45 road hub has POE of 45. This gives theoretical 8 degrees before system engages. In practice, most of the time, it will differ, as the gear ratio must be accounted for. The effective degree, as it is experienced by a rider on the cranks, is determined by the gear ratio. Let us review two examples. A road gear ratio with 50T chainring and 11T sprocket. Effective degree equals to 8 divided by 4.54. The latter is the gear ratio of 50 divided by 11. This gives as 1.76. If we take 28T chainring and 45T sprocket, MTB gear ratio of 0.62, we will get effective degree of 12.86. At the same time to evaluate these numbers, one need to give them a perspective. Compare it with a hub having POE of 18, and therefore theoretical 20 degrees. With the same gear ration of 0.62, effective angle changes to 32.12 degrees. Or if we translate that into the distance that the average 175 mm crank’s arm travels, before the power gets transmitted, it will be 56 mm. This is large enough to notice and would feel different from mere 22 mm of the system with Chris King’s R45 hub. These examples show that high POE hubs can make a big difference when the gear ratio is less than 1, such us when tackling steep off-road climbs.
The POE term comes from the most widely spread engagement design – pawls and a toothed ring. It has been around for decades. This system uses several steel pawls, commonly integrated with the freehub, and a ratchet ring coupled with the hub shell. In some types, like Race Face’s Vault hub, Rotaz or now the legacy American Classic, the pawls are in the hub shell, whilst the ratchet ring is attached to the freehub. Yet, the principle remains the same. Each pawl has one or several teeth, that when meshed with the teeth of the ratchet ring transfer the rotational motion in one direction. When moving on the opposite direction the teeth of the pawls and of the ratchet ring slide up against each other. It is what makes that “clicking” sound of the hub in coasting mode. In some pawls inspired designs, like Rotor’s Rvolver, the toothed ring is not fixed in the shell of the hub, but floats in it and the pawls have cylindrical shape.
This design has countless variants with the number of pawls ranging from two to eight, and POE ranging from mediocre 18 up to the overwhelming 690 on the latest Industry Nine Hydra MTB hub. Or translated into angles and degrees from 20 to 0.52.
Continue to Part 2